Sparsity and cold start problem are two classic problems of collaborative filtering. To alleviate these issues, researchers usually add side information to the recommendation models to boost the performance. In this paper, we propose a multi-task learning approach for recommendation based on knowledge graph (KGeRec), which takes recommendation as the main task and the knowledge graph as an auxiliary task to provide side information for recommendation. To fully capture the correlation information between these two tasks, a feature interaction layer (FlU) based on cross networks is designed to share features between them. Besides, a side information embedding layer (SIE) is also designed in the recommendation task to exploit more feature information. We apply KGeRec to three public datasets about movie, book, and music. Experimental results show that the proposed KGeRec outperforms the state-of-the-art approaches (+2.2% in AUC, +2.6% in Accuracy, +2.5% and in F1-score, compared to the maximum value in Type I models; +1.3% in AUC, +0.8% in Accuracy, and +2% in F1-score, compared to the maximum value in Type II models) and it performs well in sparse datasets. We also validate the effectiveness of knowledge graphs in improving recommendation performance.